Conventionally, spectacle lenses are manufactured on request in accordance with specifications intrinsic to individual wearers. However lenses are commonly manufactured by using a limited number of semi-finished lens blanks. A semi-finished lens blank has a front face and a rear face.
In the frame of the present invention and according to ISO Standard 13666:1998(E/F) (Ophthalmic optics-Spectacle lenses-Vocabulary), the curvature of the front face is called a “base-curve”.
The front face of a semi-finished lens blank is usually intended to be the final front surface of the final lens and the other face is machined so as the optical system of the final lens fits the wearer ophthalmic prescriptions. Some minor machining of the front face may occur, but without modifying its curvature.
Semi-finished lens blanks are usually obtained by injection moulding or by casting into moulds. They also can be produced by machining a blank.
Manufacturers typically produce a series of semi-finished lens blanks, each having its own base curve. This “base-curve series” is a system of semi-finished lens blanks that front faces increase incrementally in curvature (e.g., +0.50 D, +2.00 D, +4.00 D, and so on).
The front surface of a semi-finished lens blank of a base-curve series serves as the starting point from which the optical surface of the back surface will be calculated and the final lens be manufactured according to a wearer prescription (or focal power).
The front surfaces of the semi-finished lens blanks of a “base-curve series” may be spheres, aspheric surfaces, progressive addition surfaces.
As for an example, progressive addition lenses (PAL) may be manufactured thanks to semi-finished lens blanks with spherical or aspheric front surfaces and the progressive addition surface is machined to form the rear face of the final lens. They can also be manufactured thanks to semi-finished lens blanks with progressive addition surfaces and the rear face of the blank is machined so as to form a spherical or toric surface. It is also possible to manufacture progressive lens thanks to semi-finished lens blanks with progressive addition surfaces and to machine the rear face of the lens blank so as to obtain a second progressive addition surface and provide “dual add” PAL.
Each base-curve in a series is conventionally used for producing a range of prescription, as specified by the manufacturer. Manufacturers use base-curve selection charts that provide the recommended prescription ranges for each base-curve in the series. An example of a typical base-curve selection chart is disclosed in patent document U.S. Pat. No. 6,948,816 where the base-curve series of FIGS. 23 A to C comprises five base-curves. The selection chart indicates the unique base-curve to be chosen according to a given prescription as a function of the spherical power SPH and of the cylindrical power CYL for curing an astigmatic vision. The disclosed selection chart relates to progressive addition lenses (PAL) in which a power continuously changes between a distance portion and a near portion. The same type of selection chart is widely used for every kind of ophthalmic lenses such as for example single lenses (spherical and/or torical), bi-focal lenses, aspherical lens, PAL.
The common trend is to limit the number of base-curves of a base-curve series in order to minimize the mould number, the stocking costs and inventory requirements. A standard base-curve series comprises less or equal to twenty base-curves, as for example equal or less or equal to ten, and preferably five to eight base-curves.
Usually, a person needing to wear spectacles and having thus a prescription filled by an ophthalmologist or by another authorized eye care professional goes to the premise of an optician. The optician or the lens provider, on the basis of the aforesaid prescription, firstly determines the semi-finished lenses suiting the best the prescription and, in a second time, advises the future wearer to choose a spectacle frame amongst a limited choice of frames adapted to the pair of lenses.
Usually, the selection of the semi-finished lens is based on optical criteria, such as the wearer prescription, optical comfort and the measured parameter of the spectacle frame are used for the edging and beveling steps.
In the sense of the invention a step of cutting the lenses according to a spectacle frame shape is called “edging” and a step of forming a bevel on an external edged of the lens is called “beveling.”
Some future wearers of spectacles are incited to choose the best looking frame rather than the pair of lenses suiting the best their prescription. These future wearers can be thus frustrated for being proposed such a limited choice of frames.
One object of the present invention is to improve the situation.